Add CONTRIBUTING.md

TimothyGu (https://github.com/FFmpeg/FFmpeg/pull/153#issuecomment-143274708)
suggested doing this a while ago, but it was never done. Adding as pull
requests seem to be still coming through.
Signed-off-by: NJosh de Kock <josh@itanimul.li>

CONTRIBUTING.md

+# Note to Github users
+Patches should be submitted to the [ffmpeg-devel mailing list](https://ffmpeg.org/mailman/listinfo/ffmpeg-devel) using `git format-patch` or `git send-email`. Github pull requests should be avoided because they are not part of our review process and **will be ignored**.
+
+See [https://ffmpeg.org/developer.html#Contributing](https://ffmpeg.org/developer.html#Contributing) for more information.

README.md

@@ -45,5 +45,4 @@ GPL. Please refer to the LICENSE file for detailed information.
 
 Patches should be submitted to the ffmpeg-devel mailing list using
 `git format-patch` or `git send-email`. Github pull requests should be
-avoided because they are not part of our review process. Few developers
-follow pull requests so they will likely be ignored.
+avoided because they are not part of our review process and will be ignored.